Optically pumped, external cavity, surface-emitting semiconductor lasers (OPS-lasers) are finding favor for diverse applications such as forensic science, video displays, optical inspection, and optical pumping of fiber-lasers. One advantage of such an OPS-laser is that the emitting wavelength thereof is arbitrarily selectable over a broad range of wavelengths through the visible portion of the electromagnetic spectrum into the infrared portion of the electromagnetic spectrum. Another advantage of such a laser is that it is relatively straightforward to operate in a single longitudinal mode to provide a very high quality output beam.
A fundamental component of an OPS-laser is what is commonly termed an OPS-chip or OPS-structure. One preferred OPS-structure includes an epitaxially-grown, multilayer mirror-structure surmounted by an epitaxially-grown, semiconductor gain-structure. After the mirror-structure and gain-structure are grown, the growth substrate is etched away and the chip is bonded mirror-side down to heat-sink substrate, usually a relatively massive copper block. A diamond heat-spreader is typically located between the mirror-structure and the copper block.
An OPS-laser-resonator is usually formed between the mirror-structure of the OPS-chip and a separate conventional mirror, axially spaced-apart from the chip. The power output of the resonator is typically limited by the ability of the diamond heat-spreader and copper block to remove heat from the chip. This heat is generated by power absorbed in the gain-structure that is not extracted as laser radiation. The mirror-structure impedes the extraction of that heat. As pump power is increased, output-power of the resonator rises until heat can no longer be effectively removed, at which point, power output drops dramatically due to free-carrier absorption by the gain-structure. This is called “thermal roll-off” by practitioners of the art.
Fortunately, epitaxially grown mirror-structures, structures formed from alternating layers of gallium arsenide (GaAs) and aluminum arsenide (AlAs), can provide both high reflectivity and reasonable thermal conductivity at wavelengths between about 870 nanometers (nm) and 1100 nm. Such structures, of course are grown on a GaAs substrate. No other semiconductor systems, for example indium phosphide (InP) and gallium antimonide (GaSb), which would be used for longer wavelength OPS-structures offer such a fortunate combination. Coupled with the problem presented by the mirror-structure impeding heat extraction, is the fact that the thermal impedance of a mirror-structure increases with increasing wavelength. This is because quarter-wave optical thickness layers of the mirror-structure become physically thicker with increasing wavelength. Further, the efficiency of OPS gain-structures decreases with increasing wavelength, which increases heat generated at a given pump-power.
There is need for an efficient cooling arrangement for OPS-chips. The cooling arrangement should increase the pump-power at which thermal roll-off occurs in general. The arrangement should also facilitate use of the thicker mirror-structures and poorly conducting semiconductor materials needed for OPS-lasers operating at fundamental wavelengths longer than 1100 nm or shorter than 900 nm.